Much of security research focuses on preventing an adversary from deciphering a message's content, but there are a number of applications that motivate the more challenging goal of “covert” communications: transmitter Alice conveying information to legitimate receiver Bob while preventing a capable and attentive adversary Willie from detecting the presence of the message. Willie detects Alice's transmission by looking for deviations from the background environment; hence, covert throughput can be increased if Willie has uncertainty about that background environment, as might be caused by user mobility or time-varying jamming. Here, we consider how the dynamics of the background environment impact the throughput of covert communications. Our previous work has shown that a background environment varying a constant number of times per codeword duration can greatly increase the covert throughput. In this paper, we consider the covert throughput for all potential rates of variation of the background. Neither extreme of variation is favorable to Alice: (1) when the background environment varies too quickly relative to the codeword length, Willie can average out the variation and hence limit Alice's covert throughput; (2) when the background environment varies too slowly, Willie can estimate the current environment and again limit Alice's covert throughput. We analyze a detector at Willie that provides an upper bound (converse) to the covert throughput of Alice for all potential rates of variation. Next, we provide an explicit construction for a scheme for Alice that provides a lower bound (achievability) on the covert throughput at each rate of variation.
Much of security research focuses on preventing an adversary from deciphering a message's content, but there are a number of applications that motivate the more challenging goal of “covert” communications: transmitter Alice conveying information to legitimate receiver Bob while preventing a capable and attentive adversary Willie from detecting the presence of the message. Willie detects Alice's transmission by looking for deviations from the background environment; hence, covert throughput can be increased if Willie has uncertainty about that background environment, as might be caused by user mobility or time-varying jamming. Here, we consider how the dynamics of the background environment impact the throughput of covert communications. Our previous work has shown that a background environment varying a constant number of times per codeword duration can greatly increase the covert throughput. In this paper, we consider the covert throughput for all potential rates of variation of the background. Neither extreme of variation is favorable to Alice: (1) when the background environment varies too quickly relative to the codeword length, Willie can average out the variation and hence limit Alice's covert throughput; (2) when the background environment varies too slowly, Willie can estimate the current environment and again limit Alice's covert throughput. We analyze a detector at Willie that provides an upper bound (converse) to the covert throughput of Alice for all potential rates of variation. Next, we provide an explicit construction for a scheme for Alice that provides a lower bound (achievability) on the covert throughput at each rate of variation.
Covert communication is achieved if the transmitter (Alice) sends messages to a legitimate receiver (Bob) without detection by an attentive warden (Willie). Recently, there has been significant work on establishing the limits of such covert communication, including our work demonstrating that a cooperative jammer can greatly improve the throughput of covert communication systems. However, this previous work has considered a discrete-time model, with the implicit implication that the results will be similar on the true continuous-time model of the physical channel. In this paper, we consider covert communication over the continuous-time channel model to understand the throughput scaling for large blocklengths. For the Alice-Bob-Willie scenario, the results from the discrete-time channel generally follow. However, when a jammer is added to the environment, we demonstrate that timing offsets between Alice's signal and that of the jammer allow for the application of co-channel interference mitigation techniques at Willie's detector, which call into question whether our previous results suggesting an improvement in the covert throughput with the help of the jammer will be obtained in practice. Finally, we consider initial approaches to thwart such a detector and achieve improved throughput scaling in the presence of the jammer.
This dissertation studies covert wireless communications where a transmitter (Alice) intends to transmit messages to a legitimate receiver (Bob) such that the presence of the message is hidden from an attentive warden (Willie). Here we consider pertinent aspects of covert communications that focus on moving such systems closer to implementation. For example, previous studies use the standard discrete-time communication model when analyzing covert communications, since this is commonly assumed without loss of generality in standard communication theory. However, it is not clear that such a model captures the salient aspects of the continuous-time covert communications problem. A power detector that is optimal for the warden in a discrete-time covert communications scenario may not be optimal on a continuous- time model. Thus, it is of interest to consider this more realistic model for physical channels. After analyzing a power optimization problem using the standard discrete-time model, we move to the key part of system implementation: the instantiation in true continuous-time systems of the discrete-time models studied to this point in the literature. A key goal is to examine Willie’s detection capability on a continuous-time model and study how the limits of covert communications change from the discrete-time case. In particular, we show that detectors for Willie can benefit from the continuous-time setting and outperform detectors based on the discrete-time model; not surprisingly, this has a significant impact on the true covert throughput of the system. Nevertheless, we establish constructions such that efficient covert communications can still be achieved in a continuous-time model, and prove the fundamental limit on the covert communication rate. After considering the continuous-time problem in detail, we then turn to addressing another limitation of previous work - the requirement for an intentional jammer to facilitate efficient covert communication. Instead, we consider how to exploit a pre-existing interference source – a radar - to achieve covert communication. We establish a covert communication scheme in such an environment, and analyze the corresponding covert rate. Finally, we consider the use of a detection technique similar to that in the covert communications problem, in the area of quantized signal detection.
Consider a channel where authorized transmitter Jack sends packets to authorized receiver Steve according to a Poisson process with rate λ packets per second for a time period T. Suppose that covert transmitter Alice wishes to communicate information to covert receiver Bob on the same channel without being detected by a watchful adversary Willie. We consider two scenarios. In the first scenario, we assume that warden Willie cannot look at packet contents but rather can only observe packet timings, and Alice must send information by inserting her own packets into the channel. We show that the number of packets that Alice can covertly transmit to Bob is on the order of the square root of the number of packets that Jack transmits to Steve; conversely, if Alice transmits more than that, she will be detected by Willie with high probability. In the second scenario, we assume that Willie can look at packet contents but that Alice can communicate across an M/M/1 queue to Bob by altering the timings of the packets going from Jack to Steve. First, Alice builds a codebook, with each codeword consisting of a sequence of packet timings to be employed for conveying the information associated with that codeword. However, to successfully employ this codebook, Alice must always have a packet to send at the appropriate time. Hence, leveraging our result from the first scenario, we propose a construction where Alice covertly slows down the packet stream so as to buffer packets to use during a succeeding codeword transmission phase. Using this approach, Alice can covertly and reliably transmit O(λT) covert bits to Bob in time period T over an M/M/1 queue with service rate μ > e · λ.
Covert communication is achieved when a transmitter Al-ice can successfully transmit a message to a receiver Bob without being detected by an attentive and capable adversary Willie. Early results demonstrated the difficulty of the covert communications problem: with AWGN discrete-time channels between all parties, onlyO(n)bits can be sent innchannel uses. But it was soon recognized that uncertainty about the environment at Willie, for example, uncertainty in his own noise statistics, could allow for a positive rate:O(n)bits can be sent covertly innchannel uses. However, most covert communication results, including this promising positive rate result, have been obtained for a discrete-time communications channel. Here, we demonstrate that the assumption of a discrete-time channel is problematic when trying to exploit Willie's noise uncertainty. In particular, we demonstrate that if Alice transmitsω(T)bits in a lengthTinterval to Bob on a continuous-time channel, then there exists a detector at Willie that can detect her transmission, as the probability of false alarm and missed detectionPMD+PFA→0asT→∞. In other words, the communication is not covert, unlike the case of a discrete-time channel.
The fundamental limits of covert communication, where a message is sent from transmitter Alice to intended recipient Bob without detection by an attentive adversary warden Willie, has been considered extensively in recent years at the physical layer. The covert throughput depends critically on the warden's understanding of the characteristics of the radio environment and the type of receiver that he employs, and, as expected, the throughput increases when the warden has some uncertainty about the environment or some non-idealities in his receiver. In this paper, we consider the covert throughput when the adversary is only able to observe the medium access control (MAC) layer in a wireless communication system. In particular, given that the system has a rate ofλpackets per slot transmitted overnchannels by allowable system users, we study the allowable rateλaby covert users while maintaining covertness from an attentive warden observing the channel status in a slotted ALOHA system. We characterize performance for wardens with different abilities to discern the number of packets on a given channel, ranging from simple receivers that detect only whether there was a packet present to complicated receivers that can determine the number of packets involved in any collision, and also consider intended recipients Bob with varying abilities to perform multi-packet reception. In contrast to prior work in covert communications, the application considered motivates the consideration of results for finite (often small) observation vector lengthsnat the adversary. Numerical results are provided both to illustrate the tightness of our achievability regions for the packet transmission rate of the covert transmitters and to demonstrate the covert throughput of the system as a function ofλandn.
Covert communications, where a transmitter Alice wishes to hide the presence of her transmitted signal from a watchful adversary Willie, has been considered extensively in recent years. Those investigations have generally considered physical-layer models, where the adversary has access to a sophisticated (often optimal) receiver to determine whether a transmission has taken place, and have addressed the question of what rate can information be communicated covertly. More recent investigations have begun to consider the change in covert rate when Willie has uncertainty about the physical layer environment. Here, we move up the protocol stack to consider the covert rate when Willie is watching the medium-access control (MAC) layer in a network employing a random access MAC such as slotted ALOHA. Based on the rate of collisions and potentially the number of users involved in those collisions, Willie attempts to determine whether unauthorized (covert) users are accessing the channel. In particular, we assume different levels of sophistication in Willie's receiver, ranging from a receiver that only can detect whether there was a collision or not, to one that can always tell exactly how many packets were on the channel in the random access system. In each case, we derive closed-form expressions for the achievable covert rates in the system. The achievable rates exhibit significantly different behavior than that observed in the study of covert systems at the physical layer.